This invention relates to a method of detecting heater resistance in electron tubes and particularly to a method of detecting heater resistance in cathode ray tubes in the presence of relatively high contact resistances.
A vacuum bake is normally included as one of the processing steps for cathode ray tubes having oxide coated cathodes. The oxide coated cathodes are usually activated during the vacuum bake by applying a constant current to the cathode heaters associated therewith for a predetermined length of time. In a production run of cathode ray tubes where heater types vary from tube to tube, it is imperative that the amount of activating current applied be selected according to the nominal value of heater resistance of the tube being processed. Otherwise, a tube requiring a higher-than-applied current would not be adequately activated and a tube requiring a lower-than-applied current would be damaged.
One method of determining heater type which has been used in the past requires an operator to identify visual markings on the tube and thereafter apply the proper activation current. The principal drawback of this method is that inattentative operators could apply the wrong activation current thereby ruining the tube.
Another method comprises measuring the heater resistance in order to identify the heater type. The problem with this method is that connector contact resistances can vary so much because of the high temperature environment, that one cannot reliably distinguish between heaters of different nominal resistances.